The present invention relates to an ignition timing control apparatus for an internal combustion engine and, particularly, to an electronic advance control apparatus.
In the operation of an internal combustion engine, it takes some time after the fuel within the cylinder has been ignited by the spark and before the cylinder pressure attains a maximum following a complete combustion of the fuel. For this reason, ignition is carried out when the piston has come to a position in advance of the top dead center, i.e., an advanced position, so that combustion will be complete when the piston comes around the top dead center for producing the maximum force.
However, as the engine speed increases over a certain high speed range, the ignition timing which has been set for the lower speed causes problems, such as the reduction in the power output and over-rotation. Therefore, for the engine speed higher than a certain high speed, the degree of advance needs to be reduced, i.e., the ignition timing needs to be delayed.
There is disclosed such an ignition control apparatus in, for example, Japanese Patent Laid-open application No. 41456/81 entitled "Magnetic Ignition Control Apparatus" filed by assignee Mitsubishi Electric Corp. on Sept. 19, 1979 and published on Apr. 18, 1981, wherein the degree of advance is reduced when the engine speed increases over a certain high speed. This apparatus, however, needs a complicated circuit arrangement for providing an ignition signal based on the pulse signal which is generated in synchronization with the rotation of the engine. In addition, the above publication does not describe clearly the purpose of reducing the degree of advance in the high speed range.